The present invention relates generally to a computer-generated hologram, and more particularly to a computer-generated hologram that is relatively simple to fabricate and serves well to reconstruct a three-dimensional image of high quality at high resolution.
Holography has been developed as one of long-standing desires to record three-dimensional images on planes. Holography is a sort of technology for recording 3D image of an object making use of a laser light source of high coherence, and a 3D image display medium fabricated by holography is called a hologram. Holograms have such difficulties that they can never be fabricated unless the object to be recorded is substantially present, because they are actually produced by a process wherein laser light is directed to the object for recording its image in a photosensitive material. In addition, the object must be kept still at the wavelength level of light during recording, and so it is difficult to record objects often in motion such as flexible objects and figures. On the other hand, there is a stereogram that makes a number of two-dimensional images created by still cameras, video cameras, computer graphics (CG), etc. stereoscopically visible. The stereogram, by definition, is a medium that makes it possible for a viewer to see an image that varies with viewing directions. Given a parallax image of an object viewed from a plurality of directions, three-dimensional effects are obtainable because the image of the object can be seen depending on viewing directions.
Now widely used is a stereogram obtained with a lens array such as a lenticular or fly's eye lens located on a printing medium. The stereogram may also be fabricated by means of holography, and is called a holographic stereogram. The holographic stereogram developed to date is roughly broken down to two types, one called a two-step holographic stereogram and the other a one-step holographic stereogram after the number of recording photosteps.
The two-step holographic stereogram, true to its name, is fabricated by a two-photostep process, whose details are set forth in Patent Publication 1. Briefly speaking, the two-photostep process comprises three steps, i.e., a step (1) of providing two-dimensional images of the object to be recorded, which are viewed from a number of positions spaced away from it, a photostep (2) of dividing a first photosensitive material into a number of areas, wherein an image corresponding to a specific area of the areas provided in step (1) is holographically recorded in that area thereby preparing a first hologram, and a photostep (3) of directing reconstructing illumination light to the first hologram so that the reconstructed image is recorded in a second photosensitive material to prepare a second hologram. On the other hand, the one-step holographic stereogram, as its name implies, is fabricated by a one-photostep process, whose details are set forth in Patent Publications 2 and 3. Briefly speaking, this process comprises a step (1) of finding light rays to be radiated from a number of positions on a photosensitive material, and a photostep (2) of dividing the photosensitive material into a number of areas, wherein the light ray to be reconstructed from a specific area of the areas found in step (1) is recorded in that specific area.
Whether the two-step holographic stereogram or the one-step holographic stereogram makes it possible for the viewer to see a three-dimensional image, and so is now used as a general-purpose medium for the reconstruction of 3D images.
For the hologram that can reconstruct 3D images, there is also known a 3D image reconstructing computer-generated hologram (CGH) proposed by the inventors (see Patent Publications 4, 5, 6 and 7). In this CGH, the surface of the object is replaced by a set of point or line light sources for calculation of phases and amplitudes on the hologram plane.
For the CGH obtained by the process wherein the surface of the object is replaced by a set of point or line light sources, it is also proposed to reproduce images in color upon reconstruction by white light, as set forth in Patent Publication 8.
Patent Publication 1
JP-A 52-4855
Patent Publication 2
U.S. Pat. No. 2,884,646
Patent Publication 3
JP-A 6-266274
Patent Publication 4
JP-A 9-319290
Patent Publication 5
JP-A 11-202741
Patent Publication 6
JP-A 2001-13858
Patent Publication 7
JP-A 2001-13859
Patent Publication 8
JP-A 2000-214751
Patent Publication 9
JP-A 2002-72837
Non-Patent Publication 1
Junpei Tsuji, “Selection of Physics, Holography”, published by Shokabo Co., Ltd. (Nov. 5, 1997), pp. 33–36.
The aforesaid prior art 3D image reconstruction processes have their own merits and demerits. Print with a lens array such as a lenticular or fly's eye lens has a merit of dispensing with any holographic photostep relying upon laser light, but it has several demerits of decreased resolution and increased thickness due to the need of a physical pixel structure such as a lens array.
The two-step holographic stereogram comprises a pixel structure-free hologram plane and so has high resolution; however, it has problems in that some holographic photosteps are required with distortion in images.
The one-step holographic stereogram has no image distortion yet with more parallaxes; however, its problems are that the pixel structure is noticeable on the hologram plane and some considerable holographic photosteps result in the need of special image processing.
The three-dimensional image reconstruction CGH proposed by the inventors has several merits of higher resolution, more parallaxes, no image distortion and no need of using any holographic photostep, but there are elusive problems in that some special image processing operations (such as hidden surface removal and correction of luminance) are necessary.